Tribology is the science of the interaction of surfaces in relative motion. It includes the study of friction, lubrication and wear. It is commonly applied to the design and development of bearings. A tribometer is used to measure the performance of materials relating to friction and wear when tested under different conditions, including pressure and velocity over time.
Several different types of tribometers have been developed including a pin on disk, oscillating, four ball, ball-on disk types. The duration of tribometer tests are frequently long and become a holding point in the development of new bearing material. ASTM standards emphasize the importance of test duration to obtain precise results. One example of an ASTM standard is provided below:
ASTM D3702-94 (2009) Standard Test Method for Wear Rate and Coefficient of Friction of Materials in Self-Lubricated Rubbing Contact Using a Thrust Washer Testing Machine:
The precision of wear measurement is relatively independent of test duration or amount of wear, but the precision of wear rate (calculation) improves with test duration and amount of wear. It is generally believed that useful wear rate precision requires the selection of test duration sufficient to produce 0.1 mm (0.004 in) of wear. Test durations will often be in the 50 to 4,000-h range.
One prior art technique involves a test apparatus in which a test surface moving at a selected velocity is contacted by a test sample while applying controlled pressure to the test sample. The test method requires a relatively long run-in period prior to beginning the data collection period. This run-in period is often a necessary step to ensure accurate results in tribology measurements. After a standardized time interval of many hours or days the test sample is removed from the apparatus and weighed to determine the rate of wear at the selected pressure and velocity. The test may include different intervals of constant velocity and pressure, and the difference in weight of the test sample from the beginning of the test interval to the end of the test interval indicates the amount of material lost during that test interval and thus gives the amount of wear achieved under the interval's velocity and pressure.
Similarly, another prior art technique involves a test apparatus in which a test sample contacts a rotating test surface. Again, a relatively long run-in period is required before beginning the test period. The test is conducted over a long period of time to permit the test sample to wear to an extent that is measurable reliably with a conventional measuring device. The test sample is again removed from the apparatus and the dimensions are taken to determine the rate of wear at the selected pressure and velocity. The difference in dimensions of the test sample from the beginning of the test to the end of the test indicates the amount of material lost during the test and gives the amount of wear achieved under the test parameters.
A dynamic measuring system is proposed in U.S. Pat. No. 4,966,030 in which a proximity sensor is attached to the supporting structure of the test sample to move with the test sample. The proximity sensor faces the wear surface disk and measures the change in distance between the supporting structure and the wear surface disk. A test is run in which pressure is applied to the test sample and the wear surface disk is rotated at a velocity to give the desired pressure and velocity over time. The difference in distance of the test sample from the beginning of the test to the end of the test indicates the amount of material lost during the test and gives the amount of wear achieved under the specific conditions. The system dynamically measures the change in the distance between the proximity sensor and the test surface. Measurements may be taken at various intervals while the test is conducted. The '030 patent also discloses a prior art pin-on-disk type wear testing device and method with reference to FIG. 3 of the patent.
Tribology testing in the field of lubricant testing presents different challenges than testing wear of a material. The thickness of the lubricant layer should be determined in lubricant testing. Optimal performance of a lubricant occurs when a system is operating at a hydrodynamic level in which a first test surface floats on a film of oil on a second test surface. A lubricant may also operate at a mixed level with surfaces partially touching through the film of oil and a minimum level of wear of the test surfaces. A third regime is boundary layer operation and the test surfaces are in contact that results in wear of the surfaces. There are longstanding problems with measuring the thickness of a lubricant layer dynamically and with sufficient accuracy to provide useful data relating to lubricant performance.
The apparatus and methods disclosed by applicant are intended to improve upon the prior art as described above. Applicant's reference to certain prior art techniques should not be construed as a representation that these are the only apparatus or methods or even that they are the most similar techniques.